take a look at the pic, it's of the upper intake manifold on my S4 N/A, Last time I had the upper intake manifold off which was about 3 months ago I must have not plugged this sensor in correctly, since then I have been experiencing worse gas mileage, a few idle problems for the first few minutes of warm starts, ect.
just wondering what the significance of this sensor is, and what problems will it cause if not plugged in.

 

its an air temperature sensor. without it plugged in, the ecu assumes i think like 68 degrees all the time and could account for your worse mileage; wich is probably the only harm it will do if not plugged in. i would plug it in right, and clear the ecu to make sure it starts reading it again.